1. Field of the Invention
The present invention relates to a brake control apparatus for a vehicle.
2. Description of the Related Art
Recently, there has been developed a brake control technique which is applied to a motor-driven vehicle using a motor as a power source for driving driven wheels or to a so-called hybrid vehicle using a motor and an internal combustion engine and which performs regeneration-coordinative brake control using braking force on the basis of hydraulic pressure (hydraulic braking force; frictional braking force) as well as regenerative braking force generated by the motor.
A brake apparatus for a vehicle which performs the above-described regeneration-coordinative brake control is generally designed to adjust the regenerative braking force and the hydraulic braking force in such a manner that the total braking force (entire braking force acting on the vehicle), which is the sum of regenerative braking force acting on driven wheels (e.g., front wheels) and hydraulic braking force (specifically, hydraulic braking force acting on the driven wheels and hydraulic braking force acting on non-driven wheels (e.g., rear wheels)) coincides with a target braking force determined on the basis of an operation input applied to a brake pedal (brake pedal depressing force or the like). See, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-174703. By virtue of the adjustment, the characteristic of the total braking force with respect to the operation input acting on the brake pedal coincides with the target characteristic; therefore, a driver does not experience an unnatural sensation during braking operation.
Moreover, such a brake apparatus for a vehicle is designed to adjust the regenerative braking force and the hydraulic braking force in such a manner that the distribution of braking force between the front wheels and the rear wheels (hereinafter called the “front-rear braking force distribution”) basically coincides with a predetermined target distribution (e.g., a so-called ideal braking force distribution), and that the proportion of the regenerative braking force becomes as large as possible. This operation stabilizes the behavior of the vehicle during braking, and enables more efficient collection, to a battery, of electrical energy generated by a motor during generation of the regenerative braking force.
In particular, in the brake apparatus for a vehicle described in the publication, higher priority is placed on electrical energy collection efficiency than on the front-wheel braking force distribution. Therefore, the brake apparatus is designed in such a manner that it adjusts the regenerative braking force such that the regenerative braking force becomes greater than a braking force acting on the driven wheels when the target braking force is distributed in accordance with the target distribution (ideal braking force distribution) (hereinafter, such braking force acting on the driven wheels will be referred to as the “driven-wheel-side target distribution braking force”).
However, increasing the regenerative braking force, while placing higher priority on electrical energy collection efficiency than on the front-wheel braking force distribution, means that the driven wheels become more likely to lock. In view of this, the brake apparatus for a vehicle described in the publication is designed such that when a tendency of the driven wheels to lock (e.g., a state in which a slippage in the deceleration direction exceeds a predetermined degree) occurs in the case where the regenerative braking force is adjusted to be greater than the driven-wheel-side target distribution braking force, the brake apparatus decreases the regenerative braking force (and increases the proportion of the hydraulic braking force acting on the non-driven wheels) by an amount corresponding to the degree of the locking tendency, to thereby render the front-rear braking force distribution close to the target distribution.
Incidentally, control of the regenerative braking force is achieved by means of controlling AC power generated by the motor functioning as a generator; and, in general, quickly increasing and decreasing the magnitude of the regenerative braking force is difficult. Accordingly, even when an instruction for decreasing the regenerative braking force is issued upon detection of a locking tendency of the driven wheels, the regenerative braking force does not decrease quickly to a target value. As a result, the driven wheels may lock before the regenerative braking force decreases to the target value.